Lateral flow immunoassay devices for the detection of the presence and/or amount of a hapten in a sample are known, such as disclosed by EP291194. Due to the small size of hapten species, sandwich capture reagent type binding reactions wherein capture reagents specifically bind at two different epitopes of the analyte are not possible. Instead a competition reaction is employed wherein analyte in the sample competes with another species, typically the same analyte, for a labelled capture reagent. Typically, the lateral flow carrier comprises a reagent zone containing particulate labelled reagent and a detection zone comprising an immobilised species downstream from the reagent zone. In the absence of analyte, labelled reagent binds at the detection zone giving rise to a detectable signal. In the presence of analyte in the sample, which is typically added upstream from the reagent zone, labelled reagent is able to bind preferentially with the analyte and thus is unable to bind at the detection zone. Thus, the signal formed at the detection zone decreases as the analyte concentration increases. For a semi-quantitative yes-no visual read test, presentation of the result in this way may lead to confusion or uncertainty for the user. In order to avoid this uncertainty, manufacturers of such immunoassays have gone to the extent of printing pictures onto each device of what positive and negative results should look like, to aid in interpretation of the result by the user. Typically, the users of such assays, for example for the detection of certain drugs of abuse, would be laboratory technicians. However, the increased use of drugs of abuse has led to an increased use of such tests by untrained individuals. Thus, the ease by which the result can be interpreted is of great importance.
U.S. Pat. Nos. 5,451,504 and 5,874,216 disclose immunoassay tests for the detection of haptens wherein the signal formed at the detection zone increases as the analyte concentration increases.
There is a need to provide alternative/improved methods for detecting the presence of an analyte in a sample, wherein the signal produced increases with an increase in analyte concentration.